ListObjects Metadata Caching (#10648)

Design: https://gist.github.com/klauspost/025c09b48ed4a1293c917cecfabdf21c

Gist of improvements:

* Cross-server caching and listing will use the same data across servers and requests.
* Lists can be arbitrarily resumed at a constant speed.
* Metadata for all files scanned is stored for streaming retrieval.
* The existing bloom filters controlled by the crawler is used for validating caches.
* Concurrent requests for the same data (or parts of it) will not spawn additional walkers.
* Listing a subdirectory of an existing recursive cache will use the cache.
* All listing operations are fully streamable so the number of objects in a bucket no 
  longer dictates the amount of memory.
* Listings can be handled by any server within the cluster.
* Caches are cleaned up when out of date or superseded by a more recent one.

cmd/erasure-common.go

@@ -51,6 +51,7 @@ func (er erasureObjects) getOnlineDisks() (newDisks []StorageAPI) {
 			}
 			di, err := disks[i-1].DiskInfo(context.Background())
 			if err != nil || di.Healing {
+
 				// - Do not consume disks which are not reachable
 				//   unformatted or simply not accessible for some reason.
 				//